Electrophotographic photoreceptors have been hitherto known to include those comprising a photosensitive layer containing as a main component an inorganic photosensitive material such as selenium, zinc oxide, cadmium sulfide or silicon. These, however, can not necessarily be satisfactory in view of characteristics such as thermal stability and durability, and also have certain problems in the manufacture and handling.
On the other hand, electrophotographic photoreceptors comprising a photosensitive layer mainly composed of an organic photoconductive compound have a number of advantages that they can be manufactured relatively with ease, are inexpensive, can be handled with ease, and also commonly have superior thermal stability compared with a selenium photoreceptor. Poly-N-vinylcarbazole is most well known as the organic photoconductive compound, and a photoreceptor comprising a photosensitive layer mainly composed of a charge-moving complex formed of this poly-N-vinylcarbazole and a Lewis acid such as 2,4,7-trinitro-9-fluolenone has been already put into practical use.
Also known is a photoreceptor comprising a function-separated photosensitive layer of a laminated layer type or single layer type in which the function of carrier generation and the function of carrier transport are allotted respectively to different materials. For example, a photoreceptor comprising a photosensitive layer comprised of a carrier generation layer comprising an amorphous selenium thin layer and a carrier transport layer containing poly-N-vinylcarbazole as a main component has been already put into practical use.
The poly-N-vinylcarbazole, however, lacks in flexibility, and a coating thereof is so hard and brittle that cracks or peeling tends to occur. Hence, a photoreceptor in which this compound is used has a poor durability, and, even when a plasticizer is added so that this disadvantage can be eliminated, may have a large residual potential at the time of electrophotographic processing. This residual potential is accumulated as a result of repeated use to gradually make fog greater, resulting in a damage of copied images.
A low-molecular organic photoconductive compound usually has no film-forming properties, and hence is used in combination with a suitable binder. This compound is preferable in view of the advantage that the physical properties or photosensitive characteristics of a coating can be controlled to a certain degree by selecting kinds, composition and so forth of the binder. There, however, is a limit in the kinds of an organic photoconductive compound having a high compatibility with the binder. Actually, there are only a little kinds of binder that can be used to constitute a photosensitive layer of a photoreceptor, in particular, an electrophotographic photorecptor.
For example, 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole, as disclosed in U.S. Pat. No. 3,189,447, has a low compatibility with a binder as exemplified by polyester or polycarbonate commonly used as a material for the photosensitive layer of an electrophotographic photoreceptor. Hence, formation of a photosensitive layer by mixing it in the amount necessary for regulating electrophotographic characteristics may result in deposition of oxadiazole at temperatures higher than 50.degree. C., bringing about the disadvantage that the electrophotographic characteristics such as charge retention and sensitivity may be lowered.
On the other hand, a diarylalkane derivative, as disclosed in U.S. Pat. No. 3,820,989, has less problem of the compatibility with binders, but has a small stability to light. Hence, it has the disadvantage that use thereof in a photosensitive layer of a photoreceptor used in repetitive transfer electrophotography in which the photosensitive layer is repeatedly charged and exposed to light may result in a gradual lowering of the sensitivity of the photosensitive layer.
U.S. Pat. No. 3,274,400 and Japanese Patent Examined Publication No. 36428/1972 disclose phenothiazine derivatives of respectively different types. Both of these, however, are disadvantageous in that they have a low sensitivity and also have a low stability in repeated use.
Stilbene compounds as disclosed in Japanese Patent Publications Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 65440/1983 and No. 198043/1983 have relatively good charge retention and sensitivity, but are not satisfactory in respect of the durability in repeated use. In addition, these compounds leave an unsettled problem in respect of the sensitivity at a low potential.
Thus, under the existing circumstances, no carrier-transporting material has been discovered which has practically satisfactory characteristics in the manufacture of electrophotographic photoreceptors.